178 ELECTROCHEMISTRY 



In fact so nearly was it devoid of mineral content as to practi- 

 cally realize that elusive ideal, the "ash-free protein," for one 

 gram of the precipitate yielded less than two milligrams of ash. 

 Yet this casein, in every way in which it was tested, proved to 

 be perfectly normal. The possibility was thus indicated of esti- 

 mating the electrochemical equivalent of casein. 



Solutions of potassium caseinate were placed in a U-tube of 

 about 30 cc. capacity, 25 cc. of solution being employed in each 

 experiment. The anode consisted of a spiral of platinum wire 

 about I mm. thick and 9 cm. long, the diameter of the spiral 

 being about ^ cm. and its "pitch" about 45 degrees. The cathode 

 simply consisted of a platinum wire dipped in the fluid in the 

 other arm of the U-tube. The U-tube was provided at the 

 bottom with a 3-way stop-cock, which could either be turned so 

 as to provide fluid communication between the two arms of the 

 tube, or else sd as to permit the contents of the anodal limb to 

 escape into a receptacle. In this way it was possible to investi- 

 gate separately, if desired, the contents of each arm of the U-tube. 

 It was found that after electrolysis the fluid in the anodal arm, 

 in which the deposition of casein occurred, was practically un- 

 altered in reaction, although its casein-content was much dimin- 

 ished. In the cathodal arm, not only did the casein-content 

 diminish, but the alkalinity of the fluid was markedly increased. 



The current was led from the terminals of a 110-volt circuit 

 through a 16 candle-power lamp, a milliamperemeter, a silver 

 titration voltameter containing 30 cc. of A^/100 AgNOs, and 

 through the solution of caseinate. 



The silver was then determined by titration with a iV/100 

 ammonium thiocyanate in the presence of a constant excess of 

 nitric acid, a constant quantity (5 cc. of saturated solution^ of 

 ferric alum being employed as indicator. 



The amount of casein which had been precipitated by the 

 current was estimated by determining the refractive indices of 

 the original and of the electrolysed solutions at the same tem- 

 perature; the differences between the refractive indices divided 

 by 0.00152 yielding the decrease in the percentage of casein con- 

 tained in the solution consequent upon electrolysis.* The quan- 

 tity of solution employed was always 25 cc. Hence the decrease 



* Cf. Chap. XIV. Also T. Brailsford Robertson (24). 



